In nuclear plants such as pressurized water nuclear reactors, filters are employed to retain the impurities carried by the fluids circulating in certain parts of the plant. These impurities may be radioactive, with the result that the filter packings retaining these impurities gradually become loaded with radioactive products.
In the case of some of these circuits, therefore, it becomes necessary to arrange each of the filters in a concrete housing ensuring biological protection of the region surrounding the filter.
For example, in the case of the circuit for chemical and volumetric monitoring of the reactor coolant water (CCV circuit), filters are arranged in some parts of the circuit to retain radioactive particles which may be carried by the reactor coolant water circulating in the primary circuit. The chemical and volumetric monitoring circuit is arranged as a bypass on the primary circuit and is situated for the most part inside the nuclear auxiliaries building adjacent to the reactor building. This circuit allows a proportion of the water in the primary circuit to be withdrawn in order to carry out the purification of this water and the adjustment of the concentrations of the chemical additives which are required in the primary fluid. The chemical and volumetric monitoring circuit consequently comprises a set of purification plants, means making it possible to perform additions of chemical products in a determined quantity, and heat exchangers allowing the water to be cooled or reheated after its withdrawal or before its return to the primary circuit.
Arranged upstream of some of the components of the volumetric and chemical monitoring circuit are filters intended to collect the radioactive impurities of a certain size which are carried by the primary fluid. These filters are arranged in concrete housings, inside the nuclear auxiliaries building.
To ensure the maintenance of these filters, it is necessary to replace the filter packing when the latter is blocked or excessively filled with radioactive products.
For this purpose, the filter body has an opening in its upper part which may be closed by means of a head and the housing comprises, in its upper part, an opening located vertically in line with the opening in the filter body and passing through the concrete wall of the housing. This opening in the housing is normally closed by a plug which ensures the continuity of the biological protection. The plug is removable and comprises means for attaching hooks enabling it to be lifted or replaced in the opening of the housing.
The replacement of the filter packing is a complex procedure involving many successive steps.
First of all, the plug closing the housing must be replaced with a handling module through which tools for unscrewing the head can be introduced into the housing. The handling module must comprise a porthole and means for illuminating the interior of the housing to enable the filter head to be seen while the tools are placed in position and the unscrewing takes place. The handling module is therefore a relatively complicated unit compared with a simple closure plug.
After unscrewing the head, the opening of this head is carried out, also by using a tool inserted into the housing through an opening in the handling module.
The handling module is then removed from the opening in the housing; a lead castle is placed above this opening, the packing is withdrawn from the filter body by using a handling and lifting means associated with the lead castle and this used packing is collected inside the lead castle. The lead castle is then removed and a new packing is installed in the filter body through the opening in the housing, using a handling and lifting tool. The handling module is then replaced on the opening of the housing and the closure and the screwing of the head is carried out using tools passing through the handling module.
The handling module is then dismounted and removed and is then replaced by the closure plug of the housing.
It appears, therefore, that the replacement of the filter packing requires a series of complex and relatively long operations and the use of tools and of specific means which must be specially designed.